This invention relates generally to arrays for performing logic functions and more particularly, it relates to a bipolar programmable logic array which has a high speed of operation over a wide range of temperature variations.
As is generally known, arrays for performing logic functions are typically referred to as logic arrays or programmable logic arrays (PLAs) and have been used in recent years to replace random logic in many digital circuits. Such programmable logic arrays are especially useful in the control section of digital systems and are frequently thought of as read only storage, read only memories, or the like. The programmable logic array is a well known conventional way of using arrays of identical circuit elements to implement arbitrary logic functions in integrated circuits. Such arrays are typically arranged in either what is referred to as an AND array or plane, or what is referred to as an OR array or plane.
When a programmable logic array is implemented on an integrated circuit it becomes desirable to maintain the speed of operation of the circuit over a wide temperature range. However, charging currents are generally reduced at higher temperatures due to the fact that resistors, which determine such currents, increase in their values at the higher temperatures and thus reduce the current. As a result, this decrease in the charging current causes degradation of the AC performance or switching operations. Therefore, increased propagation delays will be experienced at the higher temperatures, thereby reducing its speed of operation.
It would therefore be desirable to provide a bipolar programmable logic array like that of the present invention which has a high speed of operation over a wide range of temperature variations and maintains a constant power dissipation.